@select macro

macro select(ex::Expr, sym1::Symbol, ex1::Expr, sym2::Symbol, ex3::Expr)
    ex.args, sym1, ex1.args, sym2, ex3.args
end

julia> @select open, high, close in  ohlcv where close > open when day=Friday
(Any[:open,:high,:(close in ohlcv)],:where,Any[:>,:close,:open],:when,Any[:day,:Friday])

# would be nice to do this instead of a one-liner

@select open, high, close
in ohlc
where open > close
when day = friday